Variations in Aggregation Structures and Fluorescence Properties of a

Jun 1, 2012 - Such differences in pressure dependence clearly reflect the degrees of PI chain packing formed at different imidization temperatures...
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Variations in Aggregation Structures and Fluorescence Properties of a Semialiphatic Fluorinated Polyimide Induced by Very High Pressure Kazuhiro Takizawa, Junji Wakita, Kenji Sekiguchi, and Shinji Ando* Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Variations in the molecular aggregation structures and optical properties of a semialiphatic fluorinated 10FEDA/ DCHM (poly(4,4′-diaminocyclohexylmethane 1,4-bis(3,4-dicarboxytrifluorophenoxy) tetrafluorobenzeneimide) polyimide (PI), which exhibits strong cyan fluorescence, were examined under very high pressure up to 8 GPa using synchrotron wide-angle X-ray diffraction (WAXD) and fluorescence spectroscopy. The fluorescence intensity of a PI film imidized at 220 °C was significantly reduced by applying pressure up to 1 GPa, which should be due to an appreciable reduction in interchain free volume, as indicated by a decrease in the d-spacing values of WAXD peaks which correspond to intermolecular ordering. In contrast, a PI film imidized at 300 °C, which exhibited weaker fluorescence than that imidized at 220 °C at atmospheric pressure, demonstrated a much smaller reduction in fluorescence intensity below 1 GPa. Such differences in pressure dependence clearly reflect the degrees of PI chain packing formed at different imidization temperatures. These phenomena induced by high pressure were almost reversible between pre- and postpressurization states with small hysteresis in the WAXD patterns and fluorescence spectra.

1. INTRODUCTION The fully aromatic polyimide (PI) derived from pyromellitic dianhydride and bis(4-aminophenyl) ether (PMDA/ODA) and that from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and pphenylenediamine (s-BPDA/PDA) are well-known super engineering plastics exhibiting high thermal and chemical stability, flame resistance, radiation resistance, mechanical strength, and good flexibility.1 Recently, fluorinated PIs and/ or semialiphatic PIs (Al-PIs) have attracted much interest as a new class of thermally stable optical material due to their colorlessness, high transparency, low refractive indices, and low birefringence.2−5 For example, they have been applied to optical waveguides, waveplates, polarizers, and peripheral components for photonics with highly controlled optical properties in the visible and near-IR regions.4 A repeating unit of PIs consists of dinahydride and diamine moieties, and it has been reported that two kinds of UV/vis absorption bands are observed for fully aromatic polyimides (Ar-PIs).6−8 The first one is a “locally excited” (LE) transition that occurs between the occupied and unoccupied molecular orbitals (MOs), both of which are located around the dianhydride moiety. The second one is a “charge transfer” (CT) transition that occurs between the dianhydride and the diamine moieties. The lowest-energy CT transition generally occurs between the highest occupied MO (HOMO) located around the diamine moiety and the lowest unoccupied MO (LUMO) located around the dianhydride moiety. The energy gap of this transition is essentially determined by the electrondonating property of diamine, as represented by its ionization © XXXX American Chemical Society

potential, and the electron-accepting property of dianhydride, as represented by its electron affinity.9 In general, a smaller energy gap in CT transition in aromatic PI films causes absorption tailing to longer wavelengths into the visible region (λ > 400 nm), which is the origin of the significant coloration of conventional Ar−PI films from pale orange to deep brown. The absorption and fluorescence properties of lowmolecular-weight imide compounds3,10,11 and aromatic/semialiphatic PIs2,3,6−8,12−18 have been studied to investigate the optical properties of PI films. Ishida et al.12 have shown that PMDA/ODA thin film exhibits strong optical absorption at 300−350 nm and absorption tailing at around 400 nm, which are attributable to LE(π−π*) and intramolecular CT absorption bands, respectively. These bands were assigned based on the absorption spectrum of a model compound: N,N′bis(phenoxyphenyl)pyromellitic imide. A weak CT fluorescence was observed at 600 nm for PMDA/ODA thin film with excitation at 520 nm.8 Hasegawa et al.3,13 reported that sBPDA/PDA exhibited broad and weak CT fluorescence at 560 nm whose excitation spectrum coincided with its absorption spectrum. They concluded that the CT fluorescence of PI is caused not only by direct excitation at the CT absorption band but also by local excitation in the BPDA moiety, followed by energy transfer from the LE to the excited CT state. Recently, we proposed a molecular design concept for highly fluorescent Received: March 10, 2012 Revised: May 22, 2012

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and polymer blends of poly[2-methoxy-5-(2′-ethylhexoxy)-pphenylenevinylene] (MEH-PPV). Guha et al.44,46 observed the fluorescence spectra and Raman scattering of polyfluorene under high pressure. The fluorescent emission of an annealed sample having a more nonplanar conformation exhibited a large pressure-induced bathochromic shift, which is explainable by the planarization of polyfluorene rings with enhanced intrachain interactions at high pressure. Recently, we reported the relationship between pressure-induced variations in the molecular aggregation structures and UV/vis absorption spectra of nonfluorescent aromatic and semialiphatic PIs up to 8 GPa.35,47 Significant variations in the LE and intermolecular CT bands were observed below 1 GPa, which accords well with a significant decrease in the interchain distance, as indicated by synchrotron WAXD patterns. This demonstrated that the UV/ vis absorption properties and the aggregation structures of PIs are closely related in a linear manner. Furthermore, by using a highly fluorescent PI film which was recently developed by the present authors, the influence of the decrease in the interchain distance of PIs at high pressures on fluorescence properties could be precisely investigated. Pressure-induced hysteresis in the aggregation structures of PIs after decreasing pressure, which was hardly observed in the UV/vis spectra,47 could also be detected by fluorescence spectra. In addition, to the best of our knowledge, variations in the interchain distance induced by high pressure have not been measured for highly fluorescent polymers by WAXD. In this study, the relationship between the molecular aggregation structures of PI chains and the fluorescence properties of a semialiphatic fluorinated 10FEDA/DCHM PI which is significantly influenced by the imidization temperature is extensively examined using synchrotron-WAXD and fluorescence spectroscopy under very high pressures up to 8 GPa.

PIs with high thermal stability as follows: (1) the use of alicylic diamines and (2) the use of aromatic dianhydrides having flexible linkages with extended conjugation.17 In particular, the use of dianhydrides whose lowest-energy transition is assignable to the n−π transition, such as PMDA and 4,4′(hexafluoroisopropylidene)diphthalic dianhydride (6FDA), should be avoided. The highly fluorescent PIs thus obtained, e.g., ODPA/DCHM prepared from 3,3′,4,4′-oxidiphthalic dianhydride and 4,4′-diaminodicyclohexylmethane, exhibit high transparency in the whole visible region and strong blue emission at 397 nm with excitation by UV light (344 nm). Our group has also reported that Al−PI synthesized from perfluorinated aromatic dianhydride, 1,4-bis(3,4dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride (10FEDA/DCHM), exhibited strong cyan fluorescence.15 On the other hand, Hasegawa et al.16 reported that the fluorescence intensity of the semialiphatic s-BPDA/CHDA prepared from sBPDA and trans-1,4-cyclohexanediamine decreased by the imidization temperature (Ti). This suggests that not only the chemical structures of the PI repeating unit but also the aggregation structures accompanying intermolecular interactions, such as π−π stacking and CT interactions, affect the optical properties of these PIs. Thereby, we consider that a detailed understanding of the aggregation structures is a prerequisite for the control of properties of PIs having optical functionalities.

2. EXPERIMENTAL SECTION Materials. 1,4-Bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride (10FEDA) obtained from NTT Advanced Technology Co., Ltd., was recrystallized from acetic anhydride and dried at 150 °C for 2 h under reduced pressure. 4,4′Diaminocyclohexylmethane (DCHM), purchased from Tokyo Kasei Kogyo Co., Ltd., was recrystallized from n-hexane, followed by sublimation under reduced pressure. The content of a trans−trans isomer in purified DCHM was estimated as 94% by 1H NMR spectrum. 35 N,O-Bis(trimethylsilyl)trifluoroacetamide (99+%, BSTFA) and N,N-dimethylacetamide (anhydrous, DMAc), purchased from Aldrich, were used without further purification. Preparation of PI Films. The structure of 10FEDA/DCHM PI is shown in Chart 1. The precursor, poly(amic acid) silyleter (PASE),

Figure 1. Photo images of a 10FEDA/DCHM PI film under white light (left) and UV (λ = 365 nm) irradiation (right).

The molecular aggregation structures of PI chains have been mainly investigated using wide-angle X-ray diffraction (WAXD).19−31 For instance, Russell et al.23 reported that the aggregation structures of PMDA/ODA films range from amorphous to highly ordered crystalline structures, depending on the film thickness and imidization conditions. In general, PI films do not exhibit definitive crystalline diffraction peaks. This indicates an absence of large domains with three-dimensional positional order. Domains with a mesomorphic order between the amorphous and crystalline phases in PI films were spontaneously formed during thermal imidization above the glass transition temperature (Tg) and frozen below Tg during cooling, which can be identified as liquid crystalline-like (LClike) ordered domains.31 Pressure is a more ideal external stimulus compared with temperature for perturbing the aggregation structures and intermolecular interactions of polymer chains in the solid state. X-ray diffraction,32−35 infrared (IR) absorption spectra,36−38 and UV/vis absorption and fluorescence spectra39−47 of polymers have been measured under high pressure to examine the compression effects on crystalline and/or amorphous structures and variations in physical properties. For instance, Drickermer et al.40,42,43 reported variations in the steady-state and time-dependent fluorescence properties of neat polymer

Chart 1. Molecular Structure of 10FEDA/DCHM PI and Optimized Geometries of PI Obtained by DFT Calculations at the Level of [B3LYP/6-311G(d)]

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was prepared by the in situ silylation method.48−50 The details of synthetic procedures for PASE have been reported elsewhere.35,47 PI films were prepared by thermal imidization of the corresponding PASE precursor. The PI films used for the pressure measurements and for the measurements at atmospheric pressure were respectively prepared by spin-coating of a PASE solution onto silica substrates and fusedsilica substrates, followed by soft baking at 70 °C for 1 h and subsequent thermal imidization in a one-step imidization procedure: the final curing conditions were 220 or 300 °C for 1.5 h. The PI films were gradually cooled to room temperature after imidization. The imidization reactions of PI films were completed in the both curing conditions (see Figure S1, Supporting Information). By controlling the spin-coating rate, two kinds of thin (ca. 4 μm thick) and thick (ca. 20 μm thick) PI films were prepared for optical measurements and WAXD measurements, respectively. The heating rate for thermal imidization was 4.6 °C/min from 70 °C to the final curing temperatures, and all curing procedures were conducted under nitrogen flow. Measurements. The fluorescence spectra of the films formed on substrates were measured at atmospheric pressure with a Hitachi F4500 fluorescence spectrometer equipped with a photomultiplier tube (Hamamatsu R928). The front-face setup was adopted for PI films to reduce the self-absorption of emitted fluorescence. The fluorescence emission spectra were measured with excitation at the peak wavelengths (λex) of corresponding excitation spectra. In contrast, the excitation spectra were measured by monitoring the fluorescence intensities at the peak wavelengths (λem) of emission spectra. The measured spectra were not corrected for the sensitivity of photomultiplier tubes to fluorescence wavelengths. Fourier-transformed infrared (FT-IR) absorption spectra were measured with a ThermoFisher Avatar-320 spectrometer equipped with a Thunderdome attenuated total reflection (ATR) attachment (incident angle was 45°). The prism (internal reflection element) was made of germanium crystal with a refractive index of 4.0. Thermomechanical analysis (TMA) was conducted with a Shimadzu TMA-60 with a fixed load of 3.0 g and a heating rate of 5 °C/min. The film thicknesses of the PIs were measured with a probe-pin type surface profilometer (DEKTAKIII). The reflection X-ray diffraction measurements were carried out in the range of 2θ = 3°−50° on an Ultima IV multipurpose X-ray diffraction system (Rigaku Co. Ltd.) with a radiation source of Cu Kα operated at 40 mA and 40 kV at a scan rate of 1°/min. The experimental procedures using a high-pressure diamond anvil cell (DAC) have been described elsewhere.47 The ruby fluorescence technique was used to estimate the pressure inside the sample room.51 The fluorescence spectra of PI films loaded in a DAC at elevated pressures were measured in the range of 200−950 nm with a multichannel CCD spectrometer (C7473-36, Hamamatsu Photonics Co., Ltd.) and objective lens (cutoff wavelength: 320 nm) for adjustment of focus. Silicone oil was used as a pressure medium, and a semiconductor laser (405 nm) was used as the light source. The unwanted emission lines were removed from the laser light using a narrow band interference filter (S76-BG3, SURUGA SEIKI Co., Ltd.). The fluorescence light emitted from a sample in the backwardscattering geometry was collected by a binary fiber equipped with a long pass filter (ITY430, 430 nm cutoff, Isuzu Glass Co., Ltd.). The resolution of the fluorescence spectra was ca. ∼0.2 nm. Because the size of the sample chamber was only 230 μm in diameter, the transmission X-ray diffraction measurements were performed with a BL40B2 beamline at the Japan Synchrotron Radiation Research Institute (SPring-8) using an image plate as the detector and silicone oil as the pressure medium. The wavelength of the X-ray was set at 0.8 Å. The diffraction peaks assignable to the silicone oil and diamond were removed from the diffraction patterns by subtracting a reference pattern measured without a sample. Quantum Chemical Calculation. The density functional theory (DFT) with a three-parameter Becke-style hybrid functional (B3LYP)52−54 was adopted for the calculations of the electronic structures of imide compounds. The 6-311G(d) basis set was utilized for the geometry optimizations.55−57 The van der Waals volumes were calculated based on Slonimski’s method58 using the optimized

geometries, in which the van der Waals radii of atoms reported by Bondi59 were used. All calculations were performed with the Gaussian03 D.0260 program package installed in the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology.

3. RESULTS AND DISCUSSION 3.1. Variations in the Aggregation Structure. A. Transmission WAXD at Atmospheric Pressure. As stated in the Introduction, the aggregation structures of PI films range from amorphous to highly ordered crystalline structures, depending on the chemical structure, film thickness, and preparation conditions. Figures 2a and 2b show the transmission and

Figure 2. (a) Transmission and (b) reflection X-ray diffraction patterns of 10FEDA/DCHM thick films (ca. 20 μm thick) imidized at 220 and 300 °C at atmospheric pressure.

reflection wide-angle X-ray diffraction (WAXD) patterns of 10FEDA/DCHM PI films (ca. 20 μm thick) which were thermally imidized at 300 °C (PI-300) and 220 °C (PI-220). The WAXD measurements were conducted at atmospheric pressure (0 GPa). The PI-300 film exhibited sharp diffraction peaks at q = 2.5 nm−1 (d = 25.2 Å) and 4.9 nm−1 (12.9 Å). These intense and sharp peaks were readily indexed as (001) and (002), respectively, because the calculated length of the PI repeating unit (L = 25.7 Å, see Chart 1) is very close to the dspacing estimated from the former peak (25.2 Å). This indicates that the periodic structure along the PI main chains (c-axis) is a pseudorod structure (see Chart 1). Because (00l) peaks were observed in both of the transmission and reflection WAXD patterns (Figures 2a and 2b), PI chains have a lower order of in-plane orientation.27 In addition, this PI exhibited diffraction peaks at q = 11.7 nm−1 (d = 5.4 Å, ch-pack-I), 13.3 nm−1 (d = 4.7 Å, ch-pack-II), 16.7 nm−1 (d = 3.8 Å), and 18.5 nm−1 (d = 3.4 Å). The present authors recently reported that semialiphatic DCHM-PIs exhibit diffraction peaks corresponding to interchain packing (ca. 5−6 Å) and π-stacking (ca. 3.6− 3.8 Å) at larger q values.31 Thereby, these peaks also represent the intermolecular orderings such as interchain packing and πC

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stacking. This PI has a liquid-crystalline-like (LC-like) domain with relatively high order because PI-300 film exhibited diffraction peaks representing the orderings along the a-, b-, and c-axes. In addition, these peaks are overlapped by broad amorphous halos ranging from 10 to 20 nm−1, which indicates that the proportion of the ordered regions is not high. The PI220 film also exhibited sharp peaks at q = 2.5 nm−1 (d = 25.2 Å) and 4.9 nm−1 (12.9 Å), but the peaks representing the intermolecular ordering were much weaker than those observed for PI-300, which indicates a lower degree of interchain ordering in the ordered domains of PI-220. We have reported that the vigorous translational motion and rearrangement of PI chains occurring during thermal treatment close to or above glass transition temperatures (Tgs) significantly enhanced the molecular packing order of PI chains.31 In addition, semialiphatic PIs whose Tgs are lower than imdization temperatures (Ti) exhibited a much higher degree of interchain ordering in the ordered domains than the PIs whose Tgs were higher than Ti. Thereby, a highly ordered domain should be formed during thermal imidization above Tg for PI-300 (Tg: 269 °C, see TMA curve in Figure S2), whereas a LC-like domain with a lower order was formed during imidization below Tg for PI-220. In the following sections, the variations in the aggregation structure of 10FEDA/DCHM PI are explained using two terms. One is the “packing structure” of the PI chains which ranges from amorphous to highly ordered crystalline structures which are significantly influenced by thermal imidization temperatures (Ti). The other one is the “intermolecular distance” which should be directly related to the interchain free volume and significantly varies by applying pressure. B. Variations in the Aggregation Structure at High Pressure. In this section, pressure-induced variations in the aggregation structure of 10FEDA/DCHM PI are investigated. Because the diffraction peaks representing the intermolecular ordering of PI-220 were too weak to analyze pressure-induced variations, only the WAXD pattern of PI-300 film was examined under very high pressure. Figure 3a shows the variations in the WAXD patterns of PI-300 film by applying pressure up to 5.8 GPa, and Figure 3b shows the magnified representations of “chpack-I” and “ch-pack-II” peaks which represent the intermolecular ordering. Figure 4a shows the variations in strain (ε) of each peak observed in Figure 3a. In this study, strain is defined as ε(hkl) = Δd/d0,(hkl), where d0,(hkl) is the d-spacing of the (hkl) plane at 0 GPa and Δd is the variation in d-spacing at elevated pressures. By applying pressure up to 5.8 GPa, both the dspacings of ch-pack-I and ch-pack-II peaks (d(ch-pack-I) and d(ch-pack-II)) were significantly decreased by 11%, whereas d(001) was decreased only by 2%. This clearly indicates that the periodic structures perpendicular to the PI main chains (aand b-axes) are more easily compressed than those along the PI main chains (c-axis). Figure 4b depicts the variations in the linear compressibilities (κ) for the diffraction peaks of ch-pack-I and -II (κch‑pack‑I and κch‑pack‑II) observed for a PI-300 film (20 μm thick). The κ values were estimated as the numerical first derivatives of strain with respect to pressure (κ = ∂ε/∂P). It should be noted that the values of κch‑pack significantly decreased from 0 up to 1 GPa, but this trend was almost completed at around 2 GPa. We have previously studied the pressure dependence of compressibility for fully aromatic s-BPDA/PDA PI and semialiphatic poly(4,4′-diaminocyclohexylmethane pyromelliticimide) (PMDA/DCHM) PI47 and reported that the easily compressible free volume in these PIs almost disappeared at 2 GPa. Hence, the large κch‑pack values observed

Figure 3. (a) Variations in the X-ray diffraction patterns for 10FEDA/ DCHM thick film imidized at 300 °C by applying pressure. (b) Magnified representation of the diffraction peaks representing the intermolecular ordering.

Figure 4. (a) Variations in the strains (ε) of the diffraction peaks of 10FEDA/DCHM thick film imidized at 300 °C by applying pressure. (b) Variations in the linear compressibilities (κ) of the diffraction peaks representing the intermolecular ordering of 10FEDA/DCHM thick film imidized at 300 °C by applying pressure.

below 2 GPa indicate the fact that semialiphatic 10FEDA/ DCHM PI contains a large amount of interchain free volume in the ordered regions, and the easily compressible free volume almost disappeared at the same pressure. Similar phenomena were observed for the pressure-induced variations in the free volume of PMMA and PC.37,38 Figure 5a displays the pressure dependence of κ(110) and κch‑pack values for 10FEDA/DCHM, s-BPDA/PDA, and D

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6 GPa. Because the main chain of PMDA/DCHM in the ordered region has an extended zigzag structure consisting of two repeating units as shown in Figure S3 of the Supporting Information, such a significant decrease in d(002) could be induced by compression stress concentrated on the bending unit of the diamine moiety, which led to drastic conformational changes along the curved main chain including the −CH2− linkage and cyclohexane structure. 3.2. Variations in Fluorescence Spectra. A. Fluorescence at Atmospheric Pressure. The intensity of the fluorescence peak for 10FEDA/DCHM PI significantly varies with the imidization temperature (Ti). Figure 6 shows the fluorescence

Figure 5. Variations in (a) the linear compressibilities along the interchain direction and (b) strains of (00l) peaks for 10FEDA/ DCHM (this work), PMDA/DCHM, and s-BPDA/PDA PIs.47

Figure 6. Fluorescence excitation and emission spectra for 10FEDA/ DCHM thin films (ca. 4 μm thick) imidized at 220 and 300 °C at atmospheric pressure.

PMDA/DCHM PIs.47 These values represent the linear compressibility of the PIs along the interchain direction. Because Ti of PMDA/DCHM (300 °C) was lower than its Tg (320 °C), this PI has a LC-like structure with a lower order. In contrast, the s-BPDA/PDA film supplied by Ube Industries has a highly ordered crystalline-like structure.47 As shown in the figure, PMDA/DCHM exhibited the highest κch‑pack value (10.5 GPa−1) at atmospheric pressure, which should be due to the loose chain packing formed in the LC-like structure. In comparison, 10FEDA/DCHM, having a LC-like structure with a relatively higher order, exhibited a κch‑pack value (7.4 GPa−1) between those of PMDA/DCHM and κ(110) of s-BPDA/PDA (3.8 GPa−1). This indicates that the chain packing of 10FEDA/ DCHM is denser than PMDA/DCHM containing a higher amount of amorphous region, but less dense compared to sBPDA/PDA containing a highly ordered crystalline region. The κ values significantly decreased by applying pressure, and it should be noted that these PIs exhibited similar κ values above 1 GPa. This indicates that the compressible free volume of each PI almost disappeared at 1 GPa, and the fully aromatic and semialiphatic PIs have similar dense chain packing at higher pressures. Figure 5b displays the pressure dependence of ε(00l) values for 10FEDA/DCHM, s-BPDA/PDA, and PMDA/DCHM PIs, which represent variations in the strain of the PIs along the PI chains. The value of d(004) for s-BPDA/PDA decreased only by 0.8% up to 6 GPa. Since the fully aromatic s-BPDA/PDA has a pseudorigid-rod structure without bent linkages, the decrease in d(004) is mainly attributable to shortening in bond lengths.47 On the other hand, the d(001) value of semialiphatic 10FEDA/DCHM having a pseudorod structure appreciably decreased by 2.3% up to 6 GPa, which could be accompanied by conformational changes around the bent ether (−O−) linkages in the dianhydride moiety plus the central −CH2− linkage and/or six-member cyclohexane structure in the diamine moiety. Moreover, the value of d(002) for semialiphatic PMDA/DCHM significantly decreased by 5.3% up to

excitation and emission spectra of PI-220 and PI-300 thin films (ca. 4 μm thick), respectively. PI-220 and PI-300 respectively showed a broad emission peak at 490 and 493 nm. The corresponding excitations are at 407 and 408 nm, respectively. As shown in the figure, the fluorescence intensity of PI-300 was significantly lower by ca. 47% compared to that of PI-220, which is explainable as follows. First, the intensity of the UV/vis absorption band at 400 nm was also lower by 20%, as shown in Figure S4 of the Supporting Information, which indicates a smaller amount of excited emissive species in PI-300. Second, the higher ordered structure formed during thermal imidization above Tg (see Figure 2) in PI-300 promoted intermolecular energy transfer between PI chains, followed by fast thermal dissipation of excitation energy, which led to nonradiation deactivation of excited species in the PI films in analogy with the concentration quenching observed for solution samples.61−63 In contrast, these deactivation processes were effectively suppressed in PI-220 due to the lower degree of interchain interactions with relatively loose chain packing. B. Variations in Fluorescence Spectra at High Pressure. The influence of the variations in the aggregation structures of PI chains on fluorescence properties was investigated under very high pressure up to 8 GPa. Figures 7a and 7b illustrate the pressure-induced variations in the fluorescence emission spectra of PI-220 and PI-300 thin films, respectively. The fluorescence intensity of each film was normalized using that of the films formed on fused silica substrates measured at atmospheric pressure (see Figure 6). The peaks were gradually shifted to longer wavelengths by 33 nm (0.16 eV) and 39 nm (0.20 eV) with increasing pressure up to 8 GPa, and their intensities significantly dropped by 97% and 94%. These pressure-induced peak shifts should be because of the reduction in the band gap due to the enhanced van der Waals interactions by decreasing interchain distance.45 The pressure-induced variations in the fluorescence peak intensities are illustrated in Figure 8. The E

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above, PI-300 contains LC-like structure with a higher order formed during thermal imidization above Tg (see Figure 6), which should enhance nonradiation deactivation of excited species. Thereby, the smaller variation in I300 below 1 GPa should be due to the higher ordered chain packing formed prior to the pressure experiment. Furthermore, the trends of significant decrease in I220 and I300 were completed at around 1 GPa, and these values gradually decreased in the same step above this pressure. This indicates that the easily compressible free volume in the 10FEDA/DCHM PI films almost disappeared at 1 GPa, and both PI-220 and PI-300 have dense and similar chain packing structures with significantly shortened interchain distance at higher pressures than 1 GPa. Figures 9a and 9b illustrate the pressure-induced hysteresis in the fluorescence spectra for PI-220 and PI-300 films after

Figure 7. Pressure dependence of fluorescence spectra for 10FEDA/ DCHM thin films imidized at (a) 220 °C and (b) 300 °C. The fluorescence intensity of each film is normalized using the values measured at atmospheric pressure (Figure 6). The peaks with asterisks (∗) arise from the fluorescence of a ruby tip for the pressure calibration.

Figure 8. Reduction of the fluorescence intensities for 10FEDA/ DCHM thin films imidized at 220 and 300 °C by applying pressure.

Figure 9. Pressure-induced hysteresis in the fluorescence spectra for 10FEDA/DCHM thin films imidized at (a) 220 °C and (b) 300 °C after releasing the pressure to 0 GPa.

peak intensities of PI-220 and PI-300 (I220 and I300) dropped by 70% and 45% up to 1 GPa. As shown in Figure S5 of the Supporting Information, the intensities of the absorption bands of PI-220 and PI-300 around the excitation wavelength (405 nm) were only slightly changed by 6% and 3% up to 1 GPa. This indicates that variations in the optical absorption induced by compression should not affect the fluorescence intensities. In contrast, the interchain distance of PI-300 was significantly decreased below 2 GPa as mentioned by WAXD (see Figure 4b). Moreover, because PI-220 consists of a LC-like ordered region and an amorphous region with a lot of free volume at atmospheric pressure (see Figure 2), the interchain free volume of PI-220 should also be significantly decreased in the first stage of compression (